1. Field of the Invention
The present invention relates to an ink jet printer, and more specifically relates to control of a conveyance roller that conveys a paper sheet.
2. Description of the Related Art
Recently, due to widespread use of personal computers and digital still cameras, printing apparatuses that utilize various methods have been developed in order to print out information produced and processed by such apparatuses. In addition, in the printing apparatuses, technology for higher recording and technology for higher print quality have been rapidly developed. In this regard, among the various types of printing apparatuses, an ink jet type serial printer that utilizes a dot matrix recording (printing) method has attracted much attention as a recording apparatus (printing apparatus) that implements printing with a higher speed and with a higher quality, and with lower costs of manufacture, as well. On the other hand, for a method of recording at a high speed in the ink jet type printing apparatus, there is a bi-directional printing method. In addition, for a method of recording with high quality in the ink jet type printing apparatus, there is a multi-pass printing method, for example.
In the ink jet printing apparatus, in order to obtain a high-quality image, it is absolutely necessary that each of a plurality of ink droplets for forming an image is jetted to be dotted at a correct position on a print medium (referred to also as a paper sheet or a recording medium) and that dots are printed in a relatively correct and accurate arrangement. However, the placement of the dots gets inaccurate due to various kinds of variance included in the printing apparatus. Further, in carrying out the bi-directional printing and the multi-pass printing, the placement of the dots gets inaccurate due to variance caused due to performance of different scanning operations for recording. Therefore, in recent printing apparatuses, a processing for aligning recording positions for aligning the placement accuracy of the dots has been a necessary technology. The recording position aligning processing is a method for aligning the positions at which the dots are formed onto the printing medium by some method. For the recording position aligning processing, there is a technology such that a correction in a main scanning direction is performed so that the dot placement position by recording scanning in a forward direction and the dot placement position by recording scanning in a return direction are matched with each other. Further, there is a method such that a correction in a sub scanning direction (that is, correction of an amount of conveyance of the print medium) is performed so that the dot placement position by a preceding recording scanning and the dot placement position by a subsequent recording scanning carried out after the print medium is conveyed are matched with each other.
A correction method in the sub scanning direction in a conventional ink jet printer is described, for example, in Japanese Patent Application Laid-Open No. 2003-011344 (corresponding to U.S. Pat. No. 6,769,759). Japanese Patent Application Laid-Open No. 2003-011344 discloses a technology such that a plurality of test patterns produced by differing the amounts of conveyance of the recording medium carried out during two pass of recording scanning are printed, then an optimum pattern is selected based on a result of printing of the test patterns, and thus a correction value of the conveyance amount is determined based on the selected test pattern. Further, Japanese Patent Application Laid-Open No. 2003-011344 discloses conveyance of paper sheet by a conveyance amount in accordance with a thus-determined correction value is carried out in performing printing.
Recently, demand for a high-quality image that is outputted from a recording apparatus with a quality as high as the quality of a photograph has been growing. Accordingly, an accuracy of conveyance of the recording medium by a conveyance roller has been improved. As the conveyance accuracy improves, it is more and more necessary that the positional alignment of the dots in the sub scanning direction be at a higher accuracy. However, in order to carry out the alignment processing in the sub scanning direction with a high accuracy, it is necessary to overcome the following drawback.
That is, as the accuracy of conveyance of the recording medium improves, an amount of slide occurring in conveying the recording medium is more accurately corrected than before. Therefore, an affect from a variance in the conveyance amount, with one rotation of the roller being a period, that occurs due to variance of an outer shape of the roller, deflection of the roller, and an attachment of a roller supporting member has been relatively high.
Here, an explanation is made as to a relationship between the affect from the variance in the conveyance amount and the image.
In this regard, the conveyance of the paper sheet is implemented by the rotation of the conveyance roller (hereinafter simply referred to also as a “roller”). For example, if a circumference of the roller is 47 mm, and when the paper sheet is conveyed by one rotation of the roller, then the paper sheet is conveyed by 47 mm.
In this regard also, when multi-pass printing for implementing high-quality printing is used, the conveyance amount in one operation of the multi-pass printing is less than a length corresponding to one rotation of the roller (47 mm). For example, the conveyance amount of the paper sheet in performing the high-quality printing is about 3.4 mm. That is, about fourteen times of sheet conveyance are carried out until the conveyance roller of the circumferential length of 47 mm fully rotates.
In this case, a variance in the conveyance amount per each phase angle, with a period being one rotation of the roller, that occurs due to the variance in the outer shape of the roller, deflection of the roller, and the attachment of a roller supporting member affects the sheet conveyance.
FIG. 6A and FIG. 6B are schematic diagrams showing a difference of sheet conveyance amount depending on the shape of the roller. If the shape of the roller is a perfect circle, and suppose that an angle of rotation of the roller for sheet conveyance is even, the conveyance amount when the roller is rotated by an angle R is the same at every position. However, when the roller has a shape different than a perfect circle, the conveyance amount when the roller is rotated by the same angle R differs depending on the rotational position of the roller. For example, if the shape of the roller is oval as shown in FIG. 6B, the sheet is conveyed in an amount L1 at a certain rotational position. Further, at another rotational position, the sheet is conveyed by an amount L2. In this case, the relationship between the length is L1>L0>L2, and thus the variance in the sheet conveyance dependent on the period of the roller occurs.
If there occurs such variance in the sheet conveyance amount dependent on the roller period, there occurs unevenness in the image recorded by the recording apparatus, and thus the quality of recording is degraded. The occurrence of the sheet conveyance amount dependent on the roller period brings about unevenness in the dot placement position of droplets, depending on the rotational position of the roller. FIG. 7A and FIG. 7B are schematic diagrams showing the unevenness. A left portion of FIG. 7A shows a roller position, and a right portion of FIG. 7A shows a direction in which the dot placement positions are deviated dependent on the roller position. In addition, FIG. 7B is a schematic diagram of a state in which the image is recorded in a state where the dot placement position is deviated. As shown in FIG. 7A, when the roller is positioned at the position L1, the sheet conveyance amount is larger than the amount of conveyance in an ordinary case, and, therefore, the image to be printed is printed in a portion lower than a position at which the actual printing is desired (an ideal position). In addition, if the roller is positioned at the position L2, the sheet conveyance amount is smaller than the conveyance amount in an ordinary case, and accordingly, the image to be printed is printed in a portion that is higher than the ideal position. Therefore, when an even image is printed, there occurs a difference in density (unevenness), as shown in FIG. 7B. The unevenness occurs much with respect to an even image such as a background portion of a scenery image, and brings a negative effect against high-quality printing.
Of course, machine accuracy of the recording apparatus has been improved in order to enable high-quality image recording. However, it is technically difficult to improve the machine accuracy to a level at which no such defect arises, and is not preferable considering a cost performance.
As described above, the variance in the outer shape of the roller causes the variance in the conveyance amount with a period of one rotation of the roller. In the same way, the deflection of the roller and the attachment of a roller supporting member brings about the variance in the conveyance amount with a period of one rotation of the roller.
Further, in the method for aligning the recording position in the sub scanning direction in which a plurality of test patterns are printed by differing the amounts of conveyance of the recording medium carried out during two passes of recording scanning, the amount of conveyance of the recording medium carried out during the recording scanning includes a variance in the conveyance due to eccentricity of the conveyance roller, in addition to the predetermined conveyance amount. In the conventional recording position aligning method, a plurality of test patterns is printed by arranging the test patterns in the sub scanning direction, and accordingly, a conveyance variance component due to the eccentricity of the conveyance roller in printing the test patterns differs in each test pattern. That is, in a method such that one of the test patterns printed in the plurality in the sub scanning direction is selected and the correction of the conveyance amount is carried out in accordance with the test pattern, the conveyance amount to be corrected includes a conveyance variance component due to the eccentricity of the conveyance roller at a predetermined position. Therefore, even if the recording position is aligned in the sub scanning direction, the recording position may not be accurately aligned in the case of conveyance in which the predetermined position of the conveyance roller is not used, thus hindering high-quality printing.